10 min read

Why This Book Matters

If you are a woman in your forties or fifties who has been exercising regularly, eating carefully, and watching your body change anyway — more belly fat, less muscle, less energy, less of everything you worked for — this book is for you. Not because it will tell you to try harder. But because it will tell you why everything you have been doing is working against your physiology, and exactly what to do instead.

Next Level was written by Stacy T. Sims, PhD, an exercise physiologist and nutrition scientist who has spent her career studying how women’s physiology differs from men’s — and how dramatically wrong most mainstream fitness advice is for women at this stage of life. Sims spent years at Stanford University and later at the University of Waikato in New Zealand researching female athletic performance. Her previous book, ROAR (2016), focused on training and nutrition around the menstrual cycle. Next Level is its sequel: everything that happens when those cycles start to end.

The book is co-written with Selene Yeager, an elite cyclist and endurance athlete who was living through perimenopause while they were writing it. That matters. This is not a theoretical text produced at clinical distance. It is written by two women who know what it feels like when the body you have trained for decades suddenly seems to be working against you — and who have the research to explain why, and what to do.

Here is the core problem the book addresses: when estrogen and progesterone begin to decline, all the physiological functions those hormones were quietly performing — building muscle, regulating blood sugar, protecting bone, managing body temperature, keeping cortisol in check — start going undone. The symptoms women experience during menopause are not random misfortunes. They are the predictable downstream effects of specific hormonal signals going offline. And the standard response most women (and most doctors) reach for — eat less, do more cardio — makes nearly all of them worse.

Sims’s prescription is specific, evidence-based, and often the opposite of conventional wisdom. That is what makes it worth reading.

The Core Framework: Picking Up the Slack

The animating concept behind Next Level is one the book introduces in the very first pages and returns to throughout: “What you’re really doing when you act on the advice in this book is picking up the slack and starting to do the work that your fluctuating and dwindling hormones have always done.”

This reframe is important. For most of a woman’s life, estrogen and progesterone have been performing anabolic, metabolic, and regulatory work in the background — stimulating muscle protein synthesis, maintaining bone density, balancing cortisol, regulating blood sugar and fat storage. You did not need to think about these functions because your hormones were handling them. As they decline, those functions do not continue automatically. The work simply goes undone unless you intervene.

Sims maps each lost hormonal function to a specific intervention:

• Estrogen’s anabolic stimulus for muscle → Heavy lifting (low reps, high load)
• Estrogen’s blood sugar regulation → Sprint interval training + strategic carbohydrate timing
• Estrogen’s mitochondrial support → Both sprint intervals and plyometrics
• Estrogen’s fast-twitch muscle and power signal → Plyometrics and heavy lifting combined
• Estrogen’s bone remodeling signal → Plyometrics and resistance training
• Progesterone’s cortisol regulation → Adequate sleep, post-workout nutrition, eliminating fasted training

This is the map. Every specific recommendation in the book flows from it.

Key Ideas

Sprint Interval Training Is the Cardio You Actually Need

The cardio most women default to during menopause — long, moderate-intensity sessions, the kind that feel virtuous and sustainable — is precisely the kind most likely to make things worse. Long steady-state cardio chronically elevates cortisol in women who already have elevated cortisol due to declining progesterone. The result is more abdominal fat storage, more muscle breakdown, and more fatigue, not less.

What works instead is sprint interval training, or SIT. Genuinely short, genuinely all-out efforts — 10 to 40 seconds — with full recovery between them. The key word is “all-out.” Not hard. Not elevated heart rate. Maximal. A Tabata protocol (20 seconds all-out, 10 seconds rest, 6-8 rounds) done on a bike or with full-body movements like kettlebell swings. Hill repeats of 20-30 seconds going as hard as possible, then walking back. This level of intensity provides the metabolic stimulus that estrogen used to provide — improving insulin sensitivity, preserving lean mass, building mitochondrial density — while the brevity of the effort prevents the chronic cortisol elevation that moderate-intensity cardio creates.

Two sessions per week is sufficient. The long run or easy bike ride does not disappear — it becomes active recovery on different days, not the primary training driver.

Lift Heavy — Not Light, Not Moderate, Heavy

The fitness industry has sold women on high-rep, low-weight training for decades, promising “toning” and “sculpting” without “bulking up.” For menopausal women, Sims is blunt: this advice is not just ineffective, it is actively unhelpful. High-rep light-weight training builds muscular endurance. Menopausal women need muscular strength.

Estrogen was the primary driver of muscle stem cell activation — the biological process that repairs and builds muscle tissue. When estrogen declines, that signal drops precipitously. Research shows that removing estrogen from animal models causes muscle stem cell regeneration to fall 30 to 60 percent. The only training that can replace this stimulus is lifting heavy enough to recruit high-threshold motor units: compound movements (squats, deadlifts, rows, chest press) performed in the 3-6 rep range at near-maximal load.

The downstream benefits extend far beyond appearance. Heavy lifting increases resting metabolic rate, improves joint stability and posture, reduces cardiovascular disease risk, strengthens bone, and produces the lean body mass that is the most significant determinant of fat metabolism in postmenopausal women. A study found that postmenopausal women had 33 percent lower fat burning during cardio than premenopausal women — and the entire difference was explained by the 9.5 pounds of lean mass they had lost.

The Cortisol Paradox: Why Eating Less Makes You Store More Fat

This is the concept that most often stops women cold when they first encounter it. They are eating less. They are exercising more. They are gaining belly fat. They are not imagining it, and they are not failures. They are caught in a cortisol paradox.

Menopausal women have elevated baseline cortisol because progesterone — the hormone that kept cortisol in check — has declined. Adding long cardio sessions (which spike cortisol), training fasted (another cortisol spike), restricting calories (which triggers metabolic survival mode), and sleeping poorly (cortisol falls 6 times more slowly in sleep-deprived people) creates a self-reinforcing stress cascade. The body interprets this cascade as survival emergency and responds accordingly: break down muscle for fuel, store abdominal fat as an energy reserve, suppress the thyroid to conserve resources.

The intervention that breaks the cycle is counterintuitive: eat enough (especially around training), replace long cardio with short intense intervals, add heavy lifting, and protect sleep. Not the “work harder, eat less” message women have been given. The opposite of it.

The 30-Minute Recovery Window and the Leucine Threshold

For muscle protein synthesis to occur, the body needs to receive a specific amino acid signal — approximately 3 to 3.5 grams of leucine per feeding — at the cellular level. This “leucine threshold” triggers the anabolic response. Meeting total daily protein without hitting the threshold at each meal does not produce the same effect.

For menopausal women, the post-workout recovery window is 30 minutes — not the 2 to 3 hours that research in male subjects suggested. After hard training (sprint intervals, heavy lifting, endurance work), cortisol is high and the body is actively breaking down muscle. Eating 30-40 grams of high-quality protein (with sufficient leucine) within that 30-minute window stops the breakdown, lowers cortisol, and initiates muscle repair. Skipping post-workout eating in an attempt to “burn more fat” does the opposite: it extends the catabolic state, elevates blood sugar through cortisol-driven glycogen release, and drives fat storage.

The practical math: 25 grams of whey protein provides about 2.5 grams of leucine. Meeting the 3-3.5 gram threshold requires 30+ grams of whey or equivalent animal protein. Plant-based athletes need roughly 50 grams of soy protein to match the leucine in 25 grams of whey — a commonly misunderstood gap.

Plyometrics for Bone Density (10 Minutes, 3x a Week)

Women can lose up to 20 percent of bone density in the five to seven years following menopause. Resistance training helps, but running and cycling — the cardio most women use — provide limited osteogenic stimulus because they involve repetitive single-plane loading rather than the multidirectional, varied-impact loading that triggers bone remodeling most effectively.

Plyometrics — jump training — fill this gap. Even 10-20 jumps twice daily has been shown in research to produce measurable improvements in hip bone density after 16 weeks. Sims recommends 10 minutes of plyometric circuits three times per week, starting with beginner movements (squat jumps, jumping jacks, side hops) and building toward more advanced options (tuck jumps, speed skaters, burpees). The investment is small. The bone density, fast-twitch muscle preservation, and insulin sensitivity benefits are significant, and there is no training category more commonly neglected by women in this age group.

Notable Quotes

“What you’re really doing when you act on the advice in this book is picking up the slack and starting to do the work that your fluctuating and dwindling hormones have always done.”

This is the book’s thesis in one sentence. Every exercise and nutrition prescription that follows is an answer to the question: which hormonal job just went undone, and how do I do it myself?

“There’s a tendency for women to lift lighter weights for high repetitions, like picking up five-pound dumbbells and lifting them 20 times. This is often called ‘body sculpting’ by trainers, who promise women that they can ‘tone up’ without ‘getting bulky muscles.’ This mindset needs to go because it’s misleading, misguided, and honestly not helpful for women whose sex hormones, lean muscle mass, and strength are on a precipitous decline.”

Sims is not gentle with the fitness industry’s treatment of menopausal women. Light weights are not a conservative starting point. They are the wrong tool for the job.

“One of the first things that happens when the body isn’t getting the energy it needs is that it starts increasing body fat. Without enough energy to perform basic functions (let alone your long runs or strength workouts), your endocrine system signals for your body to start breaking down muscle and to store more fat, so you have a reserve of energy.”

The explanation most women who are dieting and exercising and getting worse results have never heard. Not willpower failure. Survival biology.

“For menopausal women, high-intensity sprint interval training sessions can provide the metabolic stimulus to trigger the performance-boosting body composition changes that our hormones helped us achieve in our premenopausal years. The key here is the intensity.”

Intensity — not duration, not consistency, not moderate effort — is the operative word. The intensity of genuine all-out effort cannot be replicated by working “hard-ish” for longer.

“Menopausal women often reach for soy because they want the plant estrogens to relieve menopausal symptoms like hot flashes. The problem is that you need twice as much soy to provide the muscle recovery benefits of animal-based protein like whey.”

A specific, commonly misunderstood finding. Soy’s phytoestrogen content does not translate into equivalent muscle protein synthesis capacity.

“Women in their forties are still in their athletic prime. We see that in inspirational athletes like seven-time world champion Rebecca Rusch, who didn’t even start bike racing until her late twenties and is still crushing competitions in her early fifties.”

The cultural reframe the whole book rests on. Menopause is not the beginning of athletic decline. It is a transition that demands a specific response — and the response produces a body that can perform at the highest levels for decades.

Who Should Read This

Next Level is best suited for women in their forties or fifties — peri- or postmenopausal — who are already active and finding that what worked before is no longer working. If you have been training consistently, eating carefully, and watching your body composition change in the wrong direction anyway, this book explains why and tells you exactly what to change.

It is also essential reading for women entering perimenopause who want to get ahead of the transition — the interventions are most effective when started early, before significant muscle and bone loss has accumulated.

Coaches, trainers, and healthcare practitioners working with women in midlife will find it valuable for the specificity of its prescriptions. The book is more useful than most clinical resources for translating physiology into actionable programming.

It is less suited for sedentary women who are just beginning to exercise. The protocols assume a baseline level of fitness and familiarity with training concepts. A complete beginner would benefit from starting with a simpler movement foundation before implementing the sprint and lifting protocols.

Women primarily dealing with the non-fitness dimensions of menopause — hormonal symptoms, vaginal changes, cognitive shifts, MHT decisions — will find this book addresses those topics but is not the primary resource for them. The New Menopause by Mary Claire Haver is a better clinical companion for that dimension.

Related Books

ROAR — Stacy T. Sims: The predecessor to Next Level, covering training and nutrition optimization across the menstrual cycle for premenopausal women. Establishes the energy availability and nutrition timing principles that Next Level builds upon.

The New Menopause — Mary Claire Haver: The clinical complement to Next Level. Where Sims focuses on exercise and nutrition, Haver covers hormonal symptom management, HRT options, and medical decision-making. Best read together.

Good Energy — Casey Means: Covers metabolic health and blood sugar regulation from a precision medicine angle. Strong overlap with Next Level‘s nutrition content; more detailed on biomarker tracking.

Outlive — Peter Attia: Covers the exercise science of longevity with significant overlap on strength training and cardiovascular training for long-term health. Approaches similar conclusions from different research; less women-specific but broader in scope.